save a 2D array in .mat file

[mariagrazia]

Hello everybody.
I have a source program in C++ that produces a two dimensional array. Is there a way to save the result in a .mat file that can be opened in matlab? Thanks for your help

[George P]

You would likely get better help if you were to ask the people at Matlab about their file format.

[mariagrazia]

Thanks anyway. The source code is in C ++, I thought about creating a file with ofstream but the result is not readable. I will try to ask the matlab team.

[lastchance]

Why do you want to do this, @mariagrazia?

If you are doing data analysis then stay within c++. If you are plotting things then you could write a text file and read it into either matlab or python. you DON'T need the specialised .mat format.

[mariagrazia]

The goal is to get an .m file that shows the contents of the array created in C ++. The creator array as a vector of vectors, contains binary data. Is there any way to get a matlab readable file? is it correct to use ofstream?

[Ganado]

Search the web: matlab .mat file format
MathWorks explains the file format here: https://www.mathworks.com/help/pdf_doc/matlab/matfile_format.pdf

Of course, only a subset of this is needed to produce the file needed for a two-dimensional array of numbers.

Page 13 shows an example of a 2x2 numeric array. First just focus on write the 2x2 case (make sure you use ofstream with binary mode set). Then, once you study this, I imagine you can adjust it to allow for arbitrary MxN dimensions.

I thought about creating a file with ofstream but the result is not readable.

Show us what you tried and somebody might be able to explain what's wrong with it. Note that writing in a binary format is much different from just using the << stream operator. You'll need to use ofstream.write.
http://www.cplusplus.com/forum/beginner/41234/
https://stackoverflow.com/questions/49492259/writing-binary-data-to-fstream-in-c
http://www.cplusplus.com/articles/DzywvCM9/

[mariagrazia]

In c ++ I wrote some code to create bid_array starting from a particular data set (.TRC).
Then, to get a .mat file I wrote:

typedef std::basic_ofstream<char> ofstream;
ofstream outfile;
outfile.open ("datafile.m", ios::out | ios::binary);

for (auto val : bid_array) {
outfile.write(reinterpret_cast<const char *>(&val), sizeof(int));
if (outfile.bad()) {
throw std::runtime_error("Failed to write to outfile!");
}
}

but I get a file with strange characters!
Thanks you.

[seeplus]

Why can't you use the mat-file interface functions to produce the file?

When writing a file as binary, you will see strange chars when the file as viewed as a text file. That's normal and expected. To see the actual content of these files you need something like a hex viewer/editor which allows you to see/edit the file contents in hex/octal/binary etc.

Note that the typedef statement is not required.